To establish a functional nervous system, axons project long distances along established pathways to reach their proper connections. Factors that influence the growth and guidance of axons are critical for the proper development of the brain, but the mechanisms by which these factors function in vivo are not well understood. We are using the optically transparent embryonic zebrafish to study and visualize how sensory neurons are guided in their natural environment. Zebrafish sensory neurons have a stereotyped morphology with two types of axons which exhibit distinct projection patterns and behaviors. Mechanisms by which axons from one cell can exhibit such different behaviors are not known. Recently, transcription factors have been shown to partially control the specific projection patterns of some axons. In zebrafish sensory neurons, inhibition of LIM-homeodomain (LIM-HD) transcription factors causes a loss of peripheral axons, without an effect on central axons. This finding suggests that LIM-HD transcription factors act differently on the formation of central and peripheral sensory neurons. We performed a microarray to identify downstream targets of LIM-HDs. Here, I aim to understand the function of one of these targets as identified by our microarray, a transmembrane protein Calsyntenin. The in vivo function of Calsyntenin is not known, however, it has been shown to regulate the trafficking of Amyloid Precursor Protein (APP) and to mediate vesicular cargo binding to kinesin-1 in cell culture. These functions suggest that Calsyntenin may be involved in trafficking and selective targeting of neuronal components - processes that are vital for axon outgrowth and guidance. Calsyntenin is expressed during the development of the nervous system at a stage when axons are guided to their targets, however, its role in neuronal development is not known. Our preliminary data suggest that Calsyntenin exerts a specific effect on the projections of peripheral sensory axons, without an effect on central axons. This phenotype is similar to that of LIM-HD inhibition, further suggesting that Calsyntenin is a downstream target of LIM-HD. In the proposed experiments, I will further characterize the effect of Calsyntenin on sensory axon growth (Aim 1), determine whether Calsyntenin is differentially trafficked to central vs. peripheral components of sensory axons (Aim 2), and identify which functional domains of Calsyntenin are responsible for the selective peripheral axon phenotype (Aim 3). PUBLIC HEALTH RELEVANCE: Project Narrative An understanding of the complex processes involved in promoting and inhibiting neuronal axon growth, and the functions of molecules that guide axons to their correct targets will be crucial for understanding disorders of neural development. Knowledge of these mechanisms will also aid in our understanding of the conditions under which axon regeneration can occur.